Polymers containing highly hydrophilic hydrogen-bonding groups in their molecules, which are typified by poly(meth)acrylic acid and polyvinyl alcohol, are known as gas barrier polymers. Although films made of only these polymers show excellent gas (e.g., oxygen gas) barrier properties in a dry atmosphere, their hydrophilicity dramatically reduces gas (e.g., oxygen gas) barrier properties in a high humidity atmosphere. In addition, there is a problem with respect to their resistance properties to hot water; e.g., they dissolve in hot water, which limits the industrial use of them as gas barrier resins.
In order to solve these problems the present inventors propose in Document 1 (Japanese Patent Application Laid-Open No. Hei10-37180 (Claim 1)) a gas-barrier resin composition that has in its chemical structure an ester bond between poly(meth)acrylic acid and a polyalcohol, and an ionic bond between the poly(meth) acrylic acid and multivalent metal ions for the purpose of improving resistance properties of gas barrier films, which are made of a polycarboxylic acid polymer and a polyalcohol polymer, to high-temperature water vapor and hot water.
Document 1 discloses that it is possible to provide gas barrier films with resistance properties to high-temperature water vapor and hot water by constraining the ratio between the ratio of ester bonds and the ratio of ionic bonds to fall in a specific range. Furthermore Document 1 discloses the following: a method for forming an ester bond between poly(meth)acrylic acid and a polyalcohol by heat treatment; and a method for forming an ionic bond between the poly(meth)acrylic acid and multivalent metal ions by further soaking a mixture of the poly(meth)acrylic acid and polyalcohol, which has been subjected to heat treatment, in water containing a multivalent metal compound.
Document 2 (Japanese Patent Application Laid-Open No. Hei10-128923 (Claim 6)) discloses retorting treatment to be carried out in water containing a metal, wherein a retort packaging container is used which is made of a film with an ester bond formed between poly(meth)acrylic acid and a polyalcohol.
Furthermore, Document 3 (Japanese Patent Application Laid-Open No. 2000-931 (Claim 5 and Example 43)) discloses a gas barrier film with improved resistance properties to high-temperature water vapor and hot water, which is made of a polycarboxylic acid-based polymer and a polyalcohol-based polymer.
Document 3 proposes a gas barrier film obtained by applying a metal compound-containing layer on the surface of a molded material layer made of a poly(meth)acrylic acid-based polymer and a polyalcohol-based polymer, and discloses that in this way it is made possible to readily provide a gas barrier film with improved gas (e.g., oxygen gas) barrier properties, as well as with improved resistance properties to high-temperature water vapor and hot water.
All of the foregoing Document 1, Document 2 and Document 3 disclose that a multivalent metal compound is allowed to act on a mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer, and that it is thus possible to provide resistance properties to high-temperature water vapor and hot water. However, for the gas barrier resin composition or gas barrier film to have sufficient oxygen-gas barrier properties and resistance properties to high-temperature water vapor and hot water, the mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer needs to be denatured by a predetermined treatment (e.g., heat treatment). If the mixture is not sufficiently denatured, it is impossible to have sufficient oxygen-gas barrier properties and resistance properties to high-temperature water vapor and hot water to the resultant film. Moreover, molded materials made of such a mixture are not stretchable; thus, a forming method is limited. In addition, Document 3 fails to describe the influence of enlargement of a gas barrier film to its gas barrier properties. For this reason, the emergence of a molded material, which is obtained by stretching a film or sheet formed by allowing a multivalent metal compound to act on a film made of a mixture of a poly(meth)acrylic acid polymer and a polyalcohol polymer, has been long awaited.
Document 4 (Japanese Patent Application No. 2002-121246 (Claim 1)) discloses a film derived from a polycarboxylic acid-based polymer (A) and a multivalent metal compound (B), wherein the peak ratio (A1560/1700) in an infrared absorption spectrum is 0.25 or more.
Document 5 (Japanese Patent Application Laid-Open No. 2000-37822 (Claim 1)) discloses a production process for a coating film, wherein a water-based coating solution that contains at least a poly(meth)acrylic acid-based polymer, a polyalcohol-based polymer, a plasticiser and a water-based medium, is applied to at least one surface of a film to be coated to form a coated film on which a coat layer is arranged, and the coated film is stretched under heating conditions, with the plasticiser being present in the coat layer. Document 5, however, fails to describe the combination of such a coating film with a multivalent metal compound layer.
It is an object of the present invention to provide a stretch-formed multilayer container that has excellent oxygen-gas barrier properties even in a high humidity atmosphere.